1. Field of the Invention
The field of the invention is pneumatic timing devices, and more particularly, those devices used in timing relays to provide a time delay between the operation of a relay electromagnet and the actuation of some relay switch contacts. Timing relays are used in circuits for controlling industrial machines and processes of many kinds, particularly those machines and processes that are driven by one or more electrical motors.
2. Description of the Prior Art
Pneumatic timers with needle-type valves are disclosed in Haydu et al, U.S. Pat. No. 3,249,716, issued Dec. 26, 1963, and in a copending application, Ser. No. 895,728, filed Apr. 12, 1978, and assigned to the assignee of the present invention. This copending application is directed to an adjustment mechanism for controlling the axial position of a valve needle. The needle extends from a valve chamber into a passageway that opens into the interior of a bellows. The valve needle controls the rate of air flow into the bellows, thereby controlling its expansion.
Besides having a mechanical adjustment that is easily and conveniently operated, a pneumatic timer should permit an operator to easily locate a particular time setting. The scale on the typical timer provides only an approximate setting. The scale is used to make a first, coarse setting of the timer, at which the actual time delay period is observed. Additional, finer settings can then be made until the actual time delay period is exactly, or within a tolerable amount of, the desired time delay period. The fewer settings that are needed, the better is the measure of adjustability of the timer.
Adjustability can be adversely affected in the manner of backlash or hysteresis, so that a timer provides inconsistent time delays from one setting to the next. This is observed when a timing reference mark is moved away from a scale mark, and is then moved back again to that mark, and where the resulting time delay is different for the second setting than for the first. Generally speaking, this is caused by a change in the shape of the valve opening and the rate of air flow therethrough. More specifically, this can be caused by the rotation of a valve needle that is not concentrically formed around its longitudinal axis, though still within manufacturing tolerances. The shape of the valve opening can also be changed when the valve needle is shifted laterally, or tilted to a position non-parallel to the longitudinal axis of its surrounding chamber.
In a prior construction, a rectangular flange formed on a valve needle to limit rotation is separated by a narrow gap from the interior walls of the valve chamber. This allows the needle to rotate, shift or tilt, or possibly move in all of these ways, with an adverse effect on the adjustability of the device.